The object of the present invention is a method for manufacturing a heat exchange element for a film heat exchanger, wherein the element, intended for a heat exchanger transferring heat from pressurized vapor being condensed inside the elements to a liquid being evaporated on the exterior surfaces of the element, is formed from oppositely positioned plastic films which are bonded to each other at selected points in order to form ducts inside the element. In addition, the invention relates to a heat exchange element which can be manufactured by the said method.
FI lay-open print 86961 describes a film heat exchanger consisting of elements made of plastic film, wherein a liquid is directed to evaporate on the exterior surfaces of oppositely positioned elements, and the forming evaporated vapor is compressed to a higher pressure and temperature by a compressor and is directed to the interior of the elements as a heating vapor, which during the heat exchange is recondensed to liquid. The disclosed heat exchanger is suitable, for example, for the distillation of sea water and for the concentration of various solutions and suspensions.
The oppositely positioned plastic films of heat exchange elements according to FI publication 86961 are bonded to each other by linear welded seams, which divide the interior of the element into zigzagging ducts leading downwardly from the top. The purpose of these ducts is to direct the flow of vapor inside the element and to ensure the distribution of the liquid flowing on the exterior surface of the element over the entire area of the element in order to exploit effectively its heat exchange capacity. Simpler heat exchange elements for a film heat exchanger, bonded by spot welds or short linear welded seams, are additionally known from previous FI lay-open print 79948 and U.S. Pat. No. 4,585,523.
The known heat exchange elements according to the said publications are characterized in that in use they bulge under the effect of the pressurized vapor directed to their interior, whereby the exterior dimensions of the element, i.e. the height and the width of the element, are at the same time reduced. As a consequence of the expansion, there are formed in the plastic films, around the spot-like or linear seams of the element, wrinkles which cause tension peaks in the films. When, during use, a fluttering movement caused by vapor flows appears in the elements, this movement has a fatiguing effect on the films, especially in the areas of the tension peaks, gradually resulting in holes and tears forming in the films. Furthermore, during the movement of the elements, their wrinkles and sharp folds rub one against another, also furthering the forming of holes in the elements.
It is an object of the present invention to provide an option by means of which the wrinkling of the heat exchange element films is avoided during the expansion of the element. This prevents the forming of tears and holes in the element, and thereby lengthens its useful life. It is a further object of the invention to prevent the vertical and horizontal shrinking of the element in consequence to the expansion. The method for manufacturing the element according to the invention is characterized in that, during the manufacture, a permanent deformation is produced in the film forming the heat exchange surfaces of the element by stretching it between the bonding points, in order to provide leeway for the bulging upon the pressurization of the interior of the element.
The stretching is carried out according to the invention preferably by means of heat and a pressure differential between the different sides of the film. In the stretching it is thus possible to use suction acting on one side of the film or a pressurized gas, such as compressed air, pressing on the other side of the film, or suction and compressed air may act simultaneously on different sides of the film.
As a consequence of the permanent deformation caused in the film by the film stretching in accordance with the invention, during the pressurization of the interior of the element the film bulges into its stretched shape without a substantial additional stretching caused by the pressurization. In the stretching according to the invention, premolding the film, it is possible to use suitable mold pieces according to which the film yielding under the effect of heat is molded without wrinkling or folding. Thereafter the film can be expanded to its stretched shape smoothly, without wrinkling or tension peaks, and therefore the film will not tend to tear or become broken by friction. Furthermore, an element stretched in accordance with the invention has stable vertical and horizontal outer dimensions, which are not decreased during the expansion. Thereby movements in the orientation of the films of the element are avoided, as are problems in coupling the elements to dimensionally stable rigid feed and outlet duct systems for liquid and vapor.
According to one preferred embodiment of the invention, the films are brought against each other at their bonding points, whereafter the heat exchange film is stretched by directing pressure between the films, in areas between the bonding points, while the heat exchange film is heated. If hot gas, such as hot air, is used as the pressure medium, it causes both pressurization and the required heating of the film. However, the heating can also be carried out by other means, for example, by using a heating chamber, IR irradiation, or electric resistors. It is possible to use resistance heaters both for producing the deformation of the films and for bonding them.
Alternatively, the stretching of the heat exchange film can be carried out by means of heating and vacuum suction. The heating can be carried out, for example, by means of electric resistors, a heated roll, irradiation, or a hot gas or liquid. Especially preferably, a planar mold tool or a suction roll can be used, against the mold surface of which the film is sucked.
The simplest heat exchange element according to the invention is produced by bonding two oppositely positioned heat exchange films to each other. The bonding produces a bag-like element the interior of which is suitably divided into ducts by means of spot-like or linear, continuous or broken weld seams which form in the element straight or meandering vapor ducts.
Alternatively, a heat exchange element according to the invention can be formed, by bonding, from two heat exchange films and at least one support film positioned between them. The support film, which preferably consists of a plastic film stronger than the heat exchange films, increases the loadbearing capacity of the element as compared with an element made up of only heat exchange films. When the heat exchanger is used for evaporating suspensions which contain solid matter, or solutions which form a precipitate, there is left on the surfaces of the heat exchange films solid matter which increases the weight of the films and causes creep of the plastic material of the films. For this reason, up to now it has not been possible to make the heat exchange elements intended to such applications very high, the maximum dimension having been approx. 2-3 m. By contrast, the maximum dimension of an element reinforced with a support film according to the invention may be up to 10 m. At the same time, the heat exchange films can be made thinner than previously; this improves their heat exchange capacity. An element reinforced with a support film can also be strung tight so that its fluttering in the operating conditions is reduced. In an element equipped with a support film the heat exchange films are preferably plastic films having a thickness within a range of 10-100 xcexcm, and a support film suitably thicker than they may be a plastic film having a thickness preferably within a range of 30 xcexcm-1 mm.
A heat exchange element according to the invention can be reinforced not only by using a support film between the heat exchange films but also by using reinforcements of a fiber material, such as a glassfiber mat or net or a woven plastic fabric, bondable between the heat exchange films. Such a reinforcement may be, for example, integrated into a support film bonded between the heat exchange elements, or it may be placed as a reinforcement directly between the heat exchange films to be bonded to each other. The said reinforcements have the additional advantage of their low thermal expansion, in which case, owing to them, the heat exchange element retains its dimensions and remains taut in spite of a temperature increase.
A heat exchange element according to the invention for a film heat exchanger, which is intended for transferring heat from a pressurized vapor being condensed inside the element to a liquid being evaporated on the exterior surfaces of the element, and which is made up of oppositely positioned flexible plastic films bonded to each other at selected points in order to form ducts in the vapor space inside the element, is characterized in that the films which make up the heat exchange surfaces of the element have been stretched or crimped in the intervals between their bonding points in such a manner that the extensions produced in the films will provide leeway for bulging during the pressurization of the interior of the element.
Especially preferably the heat exchange element according to the invention is made up of two heat exchange films stretched or crimped between the bonding points and of at least one substantially straight support film positioned between them.